Bandgap voltage reference circuits are well known in the art. They are implemented where it is required to provide a stable voltage supply that is temperature independent over a wide range of operating temperatures. Typically they operate by combining the negative temperature coefficient of an emitter-base voltage (i.e. a CTAT or Complementary To Absolute Temperature voltage) with the positive temperature coefficient of an emitter-base voltage differential of two transistors (i.e. a PTAT or Proportional To Absolute Temperature voltage), the two transistors operating at different current densities, to make a substantially zero temperature coefficient reference voltage.
An example of one such voltage reference circuit is described in New Developments in IC Voltage Regulators, IEEE Journal of Solid-State Circuits Vol SC-6 No 1 February 1971, pages 2-7. However one of the problems associated with this traditional voltage reference circuit is that although the bandgap voltage output is independent of temperature to a first order, the output of this standard circuit is found to include a term that varies with TlnT, where T is absolute temperature and “In” is the natural logarithm function. FIG. 1 is a graph showing an example of the output voltage of such a circuit. It is apparent that the output exhibits a “bow-shape” response. This curvature indicates that the reference voltage does not remain constant over a range of temperatures and therefore fails to achieve the ideal of a temperature independent voltage reference.
A modification to overcome this problem was proposed by Jonathan M. Audy and is described in U.S. Pat. No. 5,352,973, assigned to the assignee of the present invention. In this patent Audy describes how to cancel the curvature by compensating for the TlnT term. It is achieved by adding a correction circuit to the standard bandgap implementation. FIG. 2 shows the circuit as implemented by Audy. The circuit to the right of the dotted line is a standard bandgap circuit with the two transistors Q1 and Q2 operating with PTAT current. The curvature cancellation circuit is shown to the left of the dotted line. In this circuit, transistor Qc1 is identical to Q2 in the main circuit, but it operates with constant current via the amplifier A2. It will be understood that as the two transistors Q2 and Qc1 are operating at the same base-emitter voltage, and Q2 is operating with PTAT current while Qc1 is operating at constant current, the result is a voltage between the two emitters of the form TlnT. This voltage generates a current through Rc, and this is the correction current.
While this aforementioned circuit substantially eliminates the curvature effect in the output voltage, there is one drawback associated with its implementation. It can be seen that as the correction transistor's terminals are connected to the inverting and non-inverting inputs, and the output of the operational amplifier, it clearly requires free voltage movement on each of the transistor's three terminals for operation. In a standard CMOS process generally only two types of bipolar transistors are available—a parasitic substrate bipolar transistor device with one terminal permanently connected to the substrate, and a lateral bipolar transistor device which has very poor performance. Therefore this implementation could not be directly implemented in standard CMOS.
Therefore there exists a need to provide a circuitry and method adapted to overcome this problem associated with the prior art.